1. Field of the Invention
The present invention relates to a member for semiconductor devices comprising an aluminum-silicon carbide-based composite alloy, to a method for producing it, and to a semiconductor device comprising the member.
2. Description of the Prior Art
The recent market demand for high-speed operation, large-scale integration semiconductor devices is greatly increasing. Therefore, the market has required much more increase in the thermal conductivity of heat-radiating substrates to be mounted with semiconductor chips, for the purpose of more efficiently removing the heat as generated by semiconductor chips. The market has further required the heat-radiating substrates to have a thermal expansion coefficient that is much nearer to that of the other members to be disposed adjacent to them, for the purpose of minimizing the thermal strain between the substrates and the adjacent members. The thermal expansion coefficient of silicon (hereinafter referred to as Si) and gallium arsenide (hereinafter referred to GaAs) that are generally used for semiconductor chips is 4.2.times.10.sup.-6 /.degree. C. and 6.5.times.10.sup.-6 /.degree. C., respectively. The thermal expansion coefficient of alumina ceramics that are generally used for outer frame members for semiconductor devices is around 6.5.times.10.sup.-6 /.degree. C. Therefore, it is desired that the thermal expansion coefficient of the substrates is near to those values.
With the recent increasing expansion of the application area of electronic appliances, the application area of semiconductor devices is being much more diversified. Above all, the same shall apply to the field of semiconductor power devices such as high-power AC conversion devices and frequency conversion devices. The quantity of heat from the semiconductor chips of those devices is from a few times to tens times that from semiconductor memories and microprocessors (in general, the former is, for example, tens W). Therefore, the requirements noted above for the heat-radiating substrates for those devices are extremely severe. Accordingly, the basic structure of the substrates may be, for example, as follows: First, Si semiconductor chips are mounted on a first heat-radiating substrate of an electricity-insulating aluminum nitride (hereinafter referred to as AlN in a simple manner) ceramic substrate with high thermal conductivity. Next, a second heat-radiating substrate with higher thermal conductivity of, for example, copper is disposed below the first heat-radiating substrate. Then, this is fitted to a water-cooling or air-cooling heat-radiating system. In that manner, therefore, the structure of the heat-radiating substrates is complicated. In this case, where an AlN ceramic having a thermal conductivity of around 170 W/m.multidot.K is used for the first heat-radiating substrate, the second heat-radiating substrate shall have a thermal conductivity of at least 200 W/m.multidot.K or more at room temperature, for efficient heat removal from the first substrate. In addition, in that case, the second heat-radiating substrate shall have a low thermal expansion coefficient of at most 10.times.10.sup.-6 /.degree. C., especially at most 8.times.10.sup.-6 /.degree. C., in order that its thermal expansion coefficient could be near to that of the AlN ceramic.
Especially in those of the power devices that generate a large quantity of heat in practical operation, the heat-radiating substrates themselves will often be heated at temperatures of 100.degree. C. or higher. As the case may be, therefore, the heat-radiating substrates in them are often required to still have a high thermal conductivity of at least 150 W/m.multidot.K at such high temperatures. In addition, the increase in the operation capacity of power devices requires much more efficient heat radiation from them. In that case, the size of the heat-radiating substrates on which semiconductor chips are mounted shall be inevitably enlarged. In this connection, the size of the heat-radiating surface of substrates for personal computers is at most from 20 to 40 mm square or so. As compared with those, for example, the size of the same for some high-power devices with large operation capacity is often over 200 mm square. In such large-sized substrates, it is necessary that the dimension change to be caused by temperature elevation during packaging and practical operation is small. For instance, warp or deformation of substrates, if occurred, gives some gaps between the warped or deformed substrates and radiators or fins, thereby lowering the heat radiativity of the substrates. As the case may be, semiconductor chips mounted on the warped or deformed substrates will be broken. For these reasons, ensuring good thermal conductivity at high temperatures of heat-radiating substrates is one of important themes in the art.
For such substrates, for example, Cu--W-based or Cu--Mo-based composite alloys have heretofore been used. However, the cost of the substrates of those composite alloys is high, since the materials for them are expensive, and, in addition, they are heavy. Recently, therefore, various aluminum (hereinafter referred to as Al in a simple manner) composite alloys have been used for inexpensive and lightweight substrate materials. For example, as one type of those alloys, mentioned are Al--SiC-based composite alloys comprising Al and silicon carbide (hereinafter referred to as SiC in a simple manner) as main components. The materials for the composite alloys are relatively inexpensive, and the composite alloys themselves are lightweight and have high thermal conductivity. In this connection, the density of Al and SiC alone is around 2.7 g/cm.sup.3 and around 3.2 g/cm.sup.3, respectively, and the thermal conductivity thereof is around 240 W/m.multidot.k and around 270 W/m.multidot.K, respectively. The thermal expansion coefficient of SiC alone is around 3.5.times.10.sup.-6 /.degree. C., and that of aluminum alone is 24.times.10.sup.-6 /.degree. C. Therefore, the thermal expansion coefficient of the composite alloys comprising them could be controlled within a broad range, and the composite alloys are specifically noticed in the art.
Such Al--SiC-based composite alloys and methods for producing them are disclosed in (1) JP-W-1-501489, (2) JP-A-2-243729, (3) JP-A-61-222668, and (4) JP-A-9-157773. The method in (1) comprises melting Al in a mixture of Sic and Al followed by solidifying the mixture through casting. The method in (2) and (3) comprises infiltrating Al melt into the pores of porous SiC. Of those, the method in (3) is a so-called pressure infiltration method, in which the Al melt is infiltrated into the porous SiC under pressure. The method in (4) comprises disposing a compact or a hot-pressed body of SiC and Al in a mold followed by heating it in vacuum therein at a temperature not lower than the melting point of Al for sintering it in liquid phase.
We, the present inventors have previously proposed an Al--SiC-based composite alloy in JP-A-10-335538. This has a thermal conductivity of at least 100 W/m.multidot.K, and a thermal expansion coefficient of at most 20.times.10.sup.-6 /.degree. C., and contains from 10 to 70% by weight of granular silicon carbide. This alloy is obtained according to a sintering method (which comprises preparing a mixed powder originally having the intended compositional ratio of Al--SiC, followed by sintering it). In one preferred embodiment of the alloy, aluminum carbide (hereinafter referred to as Al.sub.4 C.sub.3 in a simple manner) is dispersed in the interface between Al and SiC. For producing the alloy, powdery materials of Al and SiC are mixed in the ratio noted above, and the resulting mixture is compacted, and then sintered in a non-oxidizing atmosphere (in general, in an atmosphere which contains at least 99% by volume of nitrogen gas and has an oxygen concentration of at most 200 ppm, and which has a dew point of not higher than -20.degree. C.) at a temperature falling between 600 and 750.degree. C. According to this method, obtained is the intended composite alloy having a thermal conductivity of at least 180 W/m.multidot.K. When the compact is sintered to give the alloy, it gives a liquid phase. Even if the liquid phase formed reaches 30% or more of the total, it flows out little. Therefore, the alloy thus produced could maintain the original shape of the compact.
In addition, the inventors have also proposed previously an Al--SiC-based composite alloy to be produced through liquid-phase sintering, in JP-A-10-280082. This alloy contains Al or an Al alloy in an amount of from 5 to 80% by weight, and is deformed little after sintered. Therefore, the dimension of the sintered alloy is nearly the intended dimension thereof, or that is, net-shape alloys are obtained therein. The production method for the alloy disclosed comprises forming a layer of a substance capable of inhibiting Al elution on at least one surface of the compact of the starting powder for the alloy, while forming a layer of a mixture of the substance with another substance that promotes Al melt infiltration on the remaining surface, followed by sintering the compact in an non-oxidizing atmosphere. In the production method, the dimension change in the outer periphery of the alloy produced is reduced, and Al elution from the entire surface of the sintered body of the alloy is almost completely prevented. According to the production method disclosed in the laid-open publication, obtained are Al--SiC-based composite alloys having a thermal expansion coefficient of at most 18.times.10.sup.-6 /.degree. C. and a thermal conductivity of at least 230 W/m.multidot.K.
However, using the alloys as obtained according to some methods mentioned above for substrates for semiconductor devices is problematic in the following points. First mentioned are the problems with the alloy as obtained according to the casting method of (1). While the compact for the alloy is cooled, the components of Al and SiC segregate therein due to the density difference between them, thereby resulting in that the composition of the alloy is often not uniform. In particular, it is inevitable that the surface of the alloy is covered with a layer of Al or an Al alloy (this is hereinafter referred to as a simple term of Al-coating layer). In general, the thickness of the Al-coating layer much varies. In addition, owing to the difference in the thermal expansion coefficient between the coating layer and the SiC-alloyed inside body, thermal stress is formed in the interface between them when heat is transferred to that interface. Therefore, if the alloy having the coating layer on its surface is directly used as a substrate, thermal stress is distributed over its surface area owing to the difference in thickness of the coating layer, thereby resulting in that the substrate is warped or deformed in the stage of packaging or in the stage of practical operation. As a result, cracks are often formed in the boundaries between the substrate and the semiconductor chips or other members mounted on it. For these reasons, therefore, the coating layer must be previously removed through machining, before the alloy is formed into substrates. However, for completely removing the coating layer from the entire surface of the alloy, the inside of the alloy where hard SiC grains are combined with soft Al to give a composite structure must also be removed in some degree, since the thickness of the coating layer varies in places. Therefore, working for the removal is difficult, and the increase in the working cost is inevitable.
On the other hand, in order to surely obtain densified alloys according to the melt infiltration method of (2) or (3) noted above, an excess amount of an Al melt-infiltrating agent must be disposed in contact with the compact. In that condition, however, Al not infiltrated into the compact deposits on the outer periphery of the compact (this may be referred to as Al elution), and time-consuming labor is needed for removing the Al deposit. In case of the method described in JP-A-10-335538 mentioned above, where the compact is sintered at a temperature higher than the melting point of Al, the same phenomenon as above also occurs in some degree.
In order to solve these problems, the method of JP-A-10-280082 mentioned above may be employed, by which the Al elution could be prevented. However, in order to remove the residue of the layer formed on the compact, considerable labor is needed. Therefore, it is desired to develop a means for preventing the formation of the Al-coating layer and for preventing the Al elution on the surface of the alloy. In addition, for promoting the spontaneous infiltration of Al, a sub-component of, for example, a metal of Groups 1a and 2a is often added to Al. However, the sub-component added lowers the thermal conductivity of the resulting alloy. For example, the thermal conductivity of the alloys as prepared with the sub-component being added thereto could be at most 170 W/m.multidot.K or so at 20.degree. C. At temperatures of 100.degree. C. or higher, those alloys will inevitably have a low thermal conductivity of lower than 150 W/m.multidot.K.
The pressure melt infiltration method of (3) comprises the following steps. First, a porous SiC compact is put in a monoaxially compressible mold, and Al or an Al alloy is put on it. The resulting set is monoaxially compressed by an external force in vacuum, with Al or the Al alloy being melted, whereby the Al melt is infiltrated under pressure into the pores of the SiC compact. Next, the thus Al-infiltrated body is gradually cooled at its bottom in a gradient temperature-profiled manner. In that condition, however, while Al is solidified, it is locally led into the deep inside of the body, thereby often giving spots with no Al infiltration thereinto (these correspond to shrinkage cavities in steel casting), since the difference in the thermal expansion coefficient between the SiC phase and the Al phase in the body is great. Therefore, the method requires a complicated fine control mechanism capable of accurately controlling both the cooling temperature profile and the pressure heating program. Accordingly, the apparatus for the method is extremely expensive. In addition, for promoting the spontaneous Al infiltration, a sub-component of, for example, a metal of Groups 1a and 2a is often added to Al. However, the sub-component added lowers the thermal conductivity of the resulting alloy. For example, the thermal conductivity of the alloys as prepared with the sub-component being added thereto could be at most 170 W/m.multidot.K or so at 20.degree. C. At temperatures of 100.degree. C. or higher, those alloys will inevitably have a low thermal conductivity of lower than 150 W/m.multidot.K.
The vacuum hot pressing method of (4) is problematic in the following points. When a continuous hot-pressing device is used for the method, the Al melt must be prevented from flowing out of the mold, since the compact is hot-pressed in vacuum at a temperature not lower than the melting point of Al. In order to surely prevent the Al melt from flowing out and to obtain a desired uniform composition, an extremely expensive apparatus is needed. On the other hand, when a batch-type hot-pressing device is used, the Al melt could be prevented from flowing out in some degree. However, the cycle of charging the compact into the mold and the controlled heating program must be intermittently repeated, and this lowers the producibility. In addition, for the same reasons as those mentioned above for the pressure melt infiltration method, it is difficult to stabilize the quality of the alloys produced. According to this method, alloys having a thermal expansion coefficient of at most 10.times.10.sup.-6 /.degree. C. and a thermal conductivity of at least 200 W/m.multidot.K, both of which are indispensable for substrates, are not obtained.